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Author Topic: SWR/Power Meter for building MFJ Cub 40 meters CW QRP Transceiver Kit  (Read 3145 times)
KK4MRN
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Posts: 85




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« on: June 26, 2014, 08:44:58 AM »

I got my MFJ Cub kit in the mail.

I have a Diamond SWR/Power meter, but I do not think it will work for the MFJ Cub output power of 2 watts.
Plus, I have a Two Tinned Tunas I built in the past, so I want to measure its output too which I think is around 250 ~ 500 mW.

I am considering the MFJ MFJ-813 which is a QRP Watt Meter and SWR meter for $39.  It can measure power from 0-5 watts and SWR from 100mW to 50w.

Is it OK to hook up this radio to an OCF dipole antenna that I have for 80-6m?  Works well with my base HF rig.  This antenna would only be used for testing the radio after I build it.  I eventually would like to take QRP and QRPp radios on the road with me.  But, I would need a portable antenna I could use for 40 meters.  Maybe even a QRP antenna tuner that will work for QRP and QRPp.  I was considering the SOTA Tuner from Hendricks qrpkits.com.   Would an antenna made out of 66 feet of 22# AWG hook up wire work?





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W1JKA
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« Reply #1 on: June 26, 2014, 09:45:42 AM »

  You will have a great time with your Cub, I have three, I also have the MFJ 813 qrp meter which I only use for initial cutting and tuning and testing of my single band antennas. Yes your Cub will work fine with a OFC antenna or any other antenna designed for the band. To get the best from your Cub consider a 10 turn tune pot and a $19.00 HyperMite audio filter, match this with a simple zip cord 40m dipole and auto jump start battery and your good to go, leave your tuners/baluns at home.

   I have several of the higher end QRP portable radios and performance wise they are all the same, but the Cubs beat them all in durability, simplicity and ease of use so they are what I use for other than fair wx picnic table portable ops.
« Last Edit: June 26, 2014, 09:50:01 AM by W1JKA » Logged
KK4MRN
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Posts: 85




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« Reply #2 on: June 26, 2014, 10:21:12 AM »

Thanks for the feedback.  Yeah, I was very excited when I got the MFJ Cub kit in the mail.

The construction manual mentions that I need a "low-level signal source tuned to the radio's band of operation.".

What exactly is that and how do I get that?  I assume it is RF source around the 40 meters code portion of the band, but where can I get this source from, another transmitter like my two tinned tunas?  Please excuse ignorance.  I feel like I do not know enough to even ask the right questions.

Would an RF Probe from like qrpkits.com help in building this kit?
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W1JKA
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« Reply #3 on: June 26, 2014, 10:54:16 AM »

The RF signal source is simply a short piece of wire(noise pick up antenna) clipped to your antenna out put, this is to tune your C27, L1 and L2 receiver BPF for the loudest noise or best weak signal sensitivity while tuning through the band. Pg.32 sec. 1b in your manual.

An RF probe while handy is certainly not needed for this kind of kit. The only test gear needed is a good magnifying glass to TRIPLE check for cold solder joints, correct number of turns on toroids and correct polarity on LED.
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KK4MRN
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« Reply #4 on: July 14, 2014, 10:00:42 AM »

I have soldered the MFJ Cub.  I wound my first toroid coils.  I started the testing and alignment.   

The LED lights, I can tune and hear CW, and I hear CW at 7.040 that I confirmed by listening to the same CW on my wide receiver which is a Kenwood TH-F6A.   The transmitter appears to work too.  I changed the slug for the BFO and I heard the 11.~ frequency on my wide receiver. I do not have the radio nor manual in front of me to list the exact frequency.  And I got the right VFO frequency (around 4.940 MHz I think).   So, I know the Cub works, but I am not done.

The MFJ 813 - QRP SWR/Power Meter - does not seem to be working for me though. 

I've tried the QRP meter with the Two Tinned Tunas transmitter and the MFJ Cub for 40 meters.  I know both transmitters work because I can hear themt on my wide receiver when I press the key.  So, either I am doing something wrong, there is not enough power to make the needle move on the QRP power meter, or there is something wrong with the meter.  I hooked up the antenna jack from the transmitter to the "Tranmitter" jack on the QRP Meter.  Then I hook the "Antenna" jack on the QRP meter to the dummy load.  I move the knob around on the QRP meter.  Nothing.  After reading the manual, I think I have it hooked up right.

Even though you mentioned I do not need a frequency counter or signal source, do you have any low-cost recommendations?
n3zi does not sell frequency counter kits no more unless you are willing to buy the programmed PIC and get the rest of the parts on your own.  He has devoted his time to selling the DDS VFO now.  Not sure if you can use a DDS VFO as a regular frequency counter.
mtechnologies.com has the Almost All Digital Electronics - Frequency Display Counter Kit for $50.
I see FoxDelta.com has various frequency counters for around $32.
Hendrix QRPKITS.com has a LED digital dial, but he is on vacation for a month.
ebay has stuff from China or India for around $20, but this can take weeks or months to get it.

Looks like I will need to get some plastic alignment tools.  I found some at Electronix Express for around US$4.
mtechnologies.com has plastic or nylon alignment tools for around $6.

Well, so far it has been fun building and aligning the radio, but I want to get it right.  I probably will use the MFJ Cub to make my first contact once I think I am ready.

I have a digital multimeter.  How would I use that to determine the RF output in watts?  That is if the multimeter can read that low.  Since the dummy load would be 50 ohms, I suppose I would check the current?  Then use Ohms Law to determine the watts?  P=Isquared x R. The multimeter does not mention doing true RMS value for AC, so do this mean it checks for peak or peak-to-peak instead?  Maybe an analog multimeter would be better?

73,
Daniel, KK4MRN.
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W1JKA
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« Reply #5 on: July 14, 2014, 10:22:20 AM »

Re: KK4MRN

Good news on your Cub, as to the 813 recheck your solder joints, a cheap vom to check continuity between ant/xtmr. connectors, meter, and your push button fwd/ref switch. For top/bottom of band and 4 most used frequencies reference markings I just use end of tooth pick for a small white dot of modeler's paint after spotting from my IC-7200.

I make my plastic alignment tools from free flat or square drink swizzle sticks from the restaurant bar and file them to shape with a small nail or jeweler's file.
« Last Edit: July 14, 2014, 10:32:51 AM by W1JKA » Logged
KK4MRN
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Posts: 85




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« Reply #6 on: July 18, 2014, 09:41:47 AM »

I was wrong about the MFJ 813 QRP SWR/Power meter.  It works.  I took my ICOM IC-7410 and turned the power down as low as it can go. The owners manual for the 7410 says it can go down to 2 watts.  The MFJ 813 shows 3 watts.  I tried my other SWR/Power meter by Diamond which can be switched to 5 watts.  The Diamond meter shows 3 watts too.

My MFJ Cub does not show any indication on either meter.  Yet, I can hear on a wide receiver when I pound my Nye Viking oval key.

I tried re-aligning again using some plastic alignment tools.  Most of the tools do not fit or they strip easily.  Still no power reading on the meters.

Looks I like have to go through the steps of soldering and checking each component to make sure I soldered it correctly making sure there are no solder bridges, cold solder joints, etc.   I may not even have wounded my 2 toroids correctly.

As for RF signal source, I may create an oscillator for 7.040 MHz using a crystal I already have.  I found some circuits in the book Experimental Methods in RF Design.  I have all the components in my junk box. 

I did read in the MFJ Construction Manual that if the RF Output is low, it could be due to the toroid coils not being wound properly.
Steps 4, 5, 6 on page 37 is where I am having difficulty because whatever I change for L6/L7, the power meter does not respond.

I found this on page 40.
L6, L7 won't tune, low output: L6, L7 wrong values or defective.   C44-C48 incorrect, U5 or Q5 defective, R19 turned down.

No RF output, low output: Trimpot R19 set low, Q6/Q7 defective, L8/L9 incorrectly values, L10/L11 incorrectly wound, C53, C55-C57 incorrect values.

So, this weekend is where I will be looking.

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W1JKA
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Posts: 1619




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« Reply #7 on: July 18, 2014, 04:40:54 PM »

Re: KK4MRN

  Be sure to check your toroid coil end connections, if you didn't get all the enamel coating burned/scraped off completely you will have no readable output power. Sometimes you can just re solder the connection in place or you may have to disconnect and re scrape the ends again.
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W1AJO
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« Reply #8 on: July 18, 2014, 08:19:49 PM »

For QRP levels you need a QRP watt meter.  I have the Oak Hills Research WM-2 which is fantastic.

http://www.ohr.com/wattmeter.htm

Good luck!
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KK4MRN
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Posts: 85




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« Reply #9 on: July 23, 2014, 06:57:48 AM »

The transmitter hooked up to a dummy load shows output on my SWR/Power meters now.

I de-soldered the toroids.   Try scrapping them more.  I used my multimeter to make sure there was continuity.   I've noticed with the Cub, if you de-solder, a solder ring will come off.  This happened when i de-soldered the LED too.  I had to de-solder the LED because i did not read ahead that you're suppose to bend it.  I since re-soldered and it works.  Yet, I was not sure it was going to work because a solder ring came off when de-soldering.  Well, with the toroids, a solder ring came off too.  I have since re-soldered the toroids.  Maybe I am not de-soldering correctly.

I am ashamed to admit that the problem was with the two pre-made chokes (L8, L9) that look like resistors.  I had not solder them in.  However, the manual says the color code for the 3.3 uH L8 is orange-orange-gold-silver.  Yet, it does not look like orange-orange, maybe brown-brown.  I have a lighted magnifying glass with a bifocal magnifier - still looks brown-brown.   

Since I did the above, C40, L6, and L7 tune now.

I have since put the case on and have it by my bed side where i listen to some CW before going to sleep at night.

Now, I just got to learn CW good enough to make my first contact so I can use the MFJ on the air.

I do plan to align the radio again once I build my signal source.
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